System for removing carbon monoxide and method for removing carbon monoxide

ABSTRACT

The object of the present invention is to obtain a carbon-monoxide removing technique capable of very effectively reducing/removing carbon monoxide present at one thousand of ppm to several % in a hydrogen-rich treatment-object gas such as a reformed gas obtained by reforming of a fuel such as natural gas, methanol, etc. to a concentration of several tens of ppm (preferably 10 ppm) or lower without excessive loss of hydrogen, even when carbon dioxide, methane are co-existent For accomplishing this object, there are provided two stages of CO removers for removing carbon monoxide from a hydrogen-containing treatment-object gas, the first-stage CO remover removing a portion of the carbon monoxide by methanation thereof through a catalyst reaction, the second-stage CO remover removing the remaining portion of the carbon monoxide mainly by oxidation thereof through a further catalyst reaction involving addition of an oxidizing agent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the technique of removing carbonmonoxide contained in a hydrogen-rich reformed gas (an example of“treatment-object gas” as so referred to in the present application)such as obtained e.g. in the reforming process of hydrocarbon fuelsincluding natural gas, naphtha, kerosene, etc, or alcoholic fuels suchas methanol.

The technique to which the present application relates is characterizedthat it can remove carbon monoxide up to a concentration of ten ppm orlower. For this ability, the-technique can be suitably employed in apower generating system using e.g. solid polymer electrolyte fuel cellwhich operates at a relatively low temperature.

For the purpose of simplifying the description, the followingdescription will be made by taking a reformed gas used in a fuel cell asan example of the treatment-object gas.

2. Description of Related Art

Conventionally, with a fuel reforming apparatus using fossil fuel suchas natural gas as raw fuel, a carbon monoxide shift converter isconnected to the downstream end of the reformer so as to convert carbonmonoxide in the reformed gas into carbon dioxide by the water-gas shiftreaction, whereby the carbon monoxide concentration is reduced (removed)to 1% approximately.

On the other hand, with a fuel reforming apparatus using methanol as rawfuel, since this apparatus involves a step of the water-gas shiftreaction, the carbon monoxide concentration is reduced (removed) to 1%approximately by appropriately maintaining the operating temperature andthe water vapor ratio.

An example of an apparatus to which the reformed gas obtained above isto be fed is a polymer electrolyte fuel cell which is one type of fuelcell.

With this type of fuel cell, since it operates at a low temperaturearound about 80° C., if the reformed gas, as the fuel gas, containscarbon monoxide even by a trace amount (e.g. greater than several tensof ppm), its electrode catalyst is poisoned by the carbon monoxide,leading to significant deterioration in the cell performance. Therefore,it is necessary to reduce the carbon monoxide concentration in the fedreformed gas to less than several tens of ppm, more preferably to lessthan 10 ppm. In other words, the carbon monoxide concentration in thehydrogen-rich reformed gas needs to be reduced (removed) by a higherlevel than the conventional standard level (about 1%).

For the purpose of such relatively high level reduction of carbonmonoxide, the method thus far has proposed the following methods.

-   (a) A CO remover having a metal catalysis is provided on the    downstream of the reformer, so that with supply of air or oxygen as    an oxidizing agent, carbon monoxide contained in the reformed gas is    oxidized to be removed as carbon dioxide.-   (b) A “methanator” is provided for causing reaction between hydrogen    and carbon monoxide contained in the reformed gas, so that the    carbon monoxide is reduced to be removed as methane.

Examples of the method (a) above include the following.

-   1. “The 2nd FCDIC Fuel Cell Symposium Lecture Proceedings: 235-240    (1995)”. In this, air is mixed with the reformed gas so as to    achieve: [O₂]/[CO]=3. Then, as this mixture gas is caused to contact    Ru catalyst, carbon monoxide in the reformed gas is selectively    oxidized and removed.-   2. Japanese laid-open patent gazette: No. Hei. 7-296837:    “Reformed-Gas Supplying System”. In this, a methanol fuel reforming    system includes a methanol retriever disposed at the downstream of a    methanol reformer and also includes a carbon-monoxide oxidation    reactor (acting as a CO remover) charged with Pt-Rh catalyst    disposed at the downstream of the methanol retiever, so as to    oxidize and remove the carbon monoxide in a methanol reformed gas.

Examples of the art (b) above include the following.

-   1. Japanese laid-open patent gazette No. Hei. 6-283189: “Fuel-Cell    Power Generating System”. In this, on the downstream of a CO shift    converter, there are disposed a CO₂ adsorber and methanator having    an Ni catalyst, so that some of carbon dioxide contained in the    reformed gas is adsorbed and removed at the CO₂ adsorber and then    carbon monoxide and the remaining carbon dioxide are methanated by    the metanator to be removed as methane.

However, the above-described methods respectively have the followingproblems.

-   (a) problem with oxidation removal

In order to sufficiently remove carbon monoxide, it is necessary to addoxygen by an amount greater than 6 chemical equivalent. Then, not onlythe carbon monoxide to be removed, a great amount of hydrogen which canbe a useful fuel will be lost by combustion

-   (b) problem with removal using methanator

With this technique, if the treatment-object gas contains also carbondioxide as is the case with a reformed gas, methanation of carbondioxide, in addition to that of carbon monoxide, tends to occur withvery high likelihood For this reason, if carbon monoxide is to beremoved sufficiently while restricting loss of hydrogen due tomethanation of carbon dioxide, it is necessary to first absorb andremove the carbon dioxide also present in the reformed gas, so that thesystem required for this tends to be complicated.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve theabove-described problems, and its object is to obtain a carbon-monoxideremoving technique capable of very effectively reducing/removing carbonmonoxide present at one thousand of ppm to several % in a hydrogen-richtreatment-object gas such as a reformed gas obtained by reforming of afuel such as natural gas, methanol, etc. to a concentration of severaltens of ppm preferably 10 ppm) or lower without excessive loss ofhydrogen (with minimizing the consumption of hydrogen), even when carbondioxide, methane are co-existent.

For accomplishing this object, according to characterizing features ofthe present invention, a system for removing carbon monoxide from ahydrogen-containing treatment-object gas comprises two stages of COremovers for removing carbon monoxide, the first-stage CO removerremoving a portion of the carbon monoxide by methanation thereof througha catalyst reaction, the second-stage CO remover removing the remainingportion of the carbon monoxide mainly by oxidation thereof through afurther catalyst reaction involving addition of an oxidizing agent.

The carbon monoxide removing system of the invention includes two stagesof first CO remover and second CO remover which are disposed in thementioned order, that the treatment-object gas containing carbonmonoxide is fed first into the first CO remover and then into the secondCO remover whereby treatment-object gas having its carbon monoxidecontent removed is obtained from the second CO remover.

In the above, the removal of carbon monoxide by the first CO remover ismethanation rein oval using catalyst reaction and that by the second COremover is mainly oxidation removal using catalyst reaction involvingaddition of an oxidizing agent.

Accordingly, in removing process, at the first CO remover, by usinghydrogen present in the surrounding, methanation of carbon monoxide ispromoted for removal of then carbon monoxide, so that no oxidizing agentis required. By this catalyst reaction, a major part (more than half) ofcarbon monoxide present in the treatment-object gas may be methanated tobe removed.

Subsequently, at the second CO remover, the remaining portion of thecarbon monoxide is removed mainly through oxidation thereof by acatalyst reaction involving addition of an oxidizing agent. In thiscase, since the amount of the carbon monoxide has already been reduced,the remaining amount of carbon monoxide can be substantially entirelyremoved (to a concentration of several ppm approximately, for instance)with restricting the amount of the oxidizing agent to be added to thetreatment-object gas.

Therefore, with this carbon-monoxide removing system, it is possible torestrict the amount of the oxidizing agent required for the removal tobe smaller than the equivalent of the carbon monoxide entering the firstCO remover. As a result, treatment-object gas free from carbon monoxidemaybe obtained with limiting the amount of useful hydrogen to beconsumed in the combustion.

Such removing system as described above may be applied as it is to acase where the treatment-object gas contains a certain amount (e.g. 20%)of carbon dioxide. This is a major characterizing feature of the presentinvention.

Preferably, in the carbon-monoxide removing system described above, thefirst CO remover includes a first metal catalyst comprising one or morekinds selected from the group consisting of Ru, Pt, Rh, Pd, etc andcapable of methanating carbon monoxide and a first-catalyst reactioncondition setting mechanism for maintaining a catalyst reaction layer ofthe remover at a temperature required for methanation reaction of thecarbon monoxide by the first metal catalyst; and

the second CO remover includes a second metal catalyst comprising one ormore kinds selected from the group consisting of Ru, Pt, Rh, Pd, etc.and capable of oxidizing the carbon monoxide, a second-catalyst reactioncondition setting mechanism for-maintaining a catalyst reaction layer ofthe remover at a temperature required for the oxidation reaction of thecarbon monoxide by the second metal catalyst, and an oxidizing-agentsupplying mechanism for supplying the oxidizing agent required for theoxidation reaction with adjustment of its addition amount.

With this system, both of the reaction at the first CO remover and thatat the second CO remover involve metal catalysts, but different catalystreaction from each other.

That is to say, at the first CO remover, the first metal catalyst isemployed and the first-catalyst reaction condition setting mechanism isprovided for providing the catalyst reaction condition for causing themethanation thereof, whereby the methanation of carbon monoxide ispromoted to ensure its treatment amount.

On the other hand, at the second CO remover, the oxidizing agentrequired for the oxidation of the carbon monoxide is supplied from theoxidizing-agent supplying mechanism and also with the second-catalystreaction condition setting mechanism, the reaction between thisoxidizing agent and the carbon monoxide is effected by the second metalcatalyst. With these, the carbon monoxide, which has already beenreduced to a relatively small amount, can be removed mainly through theoxidation by the second metal catalyst.

Now, if the above-described treatment is effected on a reformed gas(such gas is obtained by reforming fuel such as hydrocarbon such asnatural gas, alcohol such as methanol, naphtha, kerosene, etc., andusually contain hydrogen more than about 50% on the dry basis) to besupplied as a fuel gas to a fuel cell, the fuel cell can operateeffectively by using the reformed gas from which carbon monoxide hasbeen removed effectively. Hence, the present invention can be suitablyapplied especially to a solid polymer electrolyte fuel cell.

In the above, the construction of the carbon monoxide removing system ofthe invention has been described. Next, there will be described theinvention's method of removing carbon monoxide using such system.

The method of removing carbon monoxide from a hydrogen-containingtreatment-object gas, according to the present invention, ischaracterized by the following steps:

a) a first step of causing the treatment-object gas to contact a firstmetal catalyst capable of methanating carbon monoxide so that a portionof the carbon monoxide is removed through its methanation; and

b) a second step of causing the treatment-object gas from the first steptogether with an oxidizing agent to contact a second metal catalystcapable of oxidizing carbon monoxide so that the remaining portion ofcarbon monoxide is removed mainly through its oxidation.

In the above, the first step corresponds to the process effected at thefirst CO remover of the invention's carbon monoxide removing systemdescribed above and the second step corresponds to the process effectedat the second CO remover of the same.

By the same operating principle as described hereinbefore in theforegoing section describing the system, with this carbon monoxideremoving method, the treatment-object gas substantially free from carbonmonoxide may be obtained, with minimizing the amount of the oxidizingagent required for removal so as to reduce the amount of useful hydrogenconsumed in the combustion. And, this removal is possible to aconcentration of several ppm or lower to several tens of ppm. Also, thisremoval is possible even when the treatment-object gas contains e.g.about 20% of carbon dioxide, without involving treatment of thiscomponent.

Preferably, in the first step, a reaction temperature of catalystreaction between a first metal catalyst and the treatment-object gas iscontrolled to a temperature at which methanation of carbon monoxide maybe promoted with restricting methanation of carbon dioxide, so as toreduce the carbon monoxide concentration of the treatment-object gas tobe obtained from this step as much as possible. In this respect, it isespecially preferred that the carbon monoxide concentration be reducedto 70% or lower, more preferably 50% or lower, most preferably 30% orlower of the original carbon concentration of the gas charged into thisfirst step. For example, if it is reduced to 50% or lower, the amount ofhydrogen loss in association with the CO oxidation-at the second stepcan be reduced and also the amount of heat generated in association withthe oxidation too can be reduced. As a result, the temperature controlof the reactor becomes easier and the CO oxidation can be effectedreliably.

With the catalyst having the methanating ability for carbon monoxide,methanation of carbon dioxide too tends to occur. Then, by restrictingthis reaction, the consumption of hydrogen may be reduced to thenecessity minimum. Moreover, by reducing the concentration of carbonmonoxide discharged from the first step to be smaller than apredetermined amount, it becomes possible to remove carbon monoxidethrough its oxidation at the second step easily and reliably and theamount of oxidizing agent required may also be reduced.

Preferably, in the first step, a first metal catalyst comprising one ormore kinds selected from the group consisting of Ru, Pt, Rh, Pd, isemployed and a catalyst reaction layer is maintained at a temperaturewhere methanation of carbon monoxide takes place by the first metalcatalyst. This is because these catalysts are capable of methanation ofcarbon monoxide.

More particularly, it is preferred that the first metal catalyst be ahigh quantity metal supported catalyst comprising the one or more kindsselected from the group consisting of Ru, Pt, Rh, and Pd by 0.1 to 5 wt.% (more preferably 0.5 to 5 wt. %) supported on a catalyst support. Inthis if the metal content is lower than 0.1 wt. %, the methanationactivity tends to be reduced. Whereas, if it exceeds 5 wt. %, nosignificant improvement in the methanation activity can be achieved.

Next, in the second step, a second metal catalyst comprising one or morekinds selected from the group consisting of Ru, Pt, Rh, and Pd isemployed and a catalyst reaction portion is maintained at a temperaturewhere oxidation of carbon monoxide takes place by the second metalcatalyst involving addition of an oxidizing agent.

The catalysts, as described hereinbefore, cause methanation of carbonmonoxide. But, at the same time, in the presence of a large amount ofoxidizing agent (oxidizing atmosphere) and at a relatively lowtemperature, they act as catalysts for mainly oxidizing carbon monoxide.Accordingly, by using such metal as the second metal catalyst suitablefor the object of the present invention, the reaction at its catalystreaction layer is controlled to be limited to an oxidation reactionmainly. Whereby, the remaining portion of the carbon monoxide maysubstantially entirely be oxidized and removed.

More particularly, it is preferred that the second metal catalyst be alow quantity metal supported catalyst comprising the one or more kindsselected from the group consisting of Ru, Pt, Rh, and Pd by 0.1 to 5 wt.% (more preferably 0.1 to 2 wt. %) supported on a catalyst support. Inthis if the metal content is lower than 0.1 wt. %, the oxidationactivity tends to be reduced. Whereas, if it exceeds 5 wt. %, nosignificant improvement in the oxidation activity can be achieved.

Further, in the above-described treatment of carbon monoxide,preferably, the total amount of the oxidizing agent supplied at thesecond step be below 3 chemical equivalents in oxygen conversionrelative to the amount of carbon monoxide originally contained in thetreatment-object gas, more preferably below 2 chemical equivalents, mostpreferably below the chemical equivalent. In this case, the consumptionamount of hydrogen may be reduced sufficiently.

Also preferably, a second-catalyst reaction temperature which is thecatalyst reaction temperature at the second step is set to be lower thana first-catalyst reaction temperature which is the catalyst reactiontemperature at the first step.

Temperature suitable for methanation exists in a relatively hightemperature range. Then, in order to cause the oxidation mainly, thisshould take place at a temperature range lower than the abovetemperature range. In such case, no heating operation becomes necessaryin particular.

Further, as described hereinbefore, preferably, the method of theinvention is applied to a reformed gas.

In the present invention, basically, the first step utilizes methanationreaction and the second step utilizes oxidation reaction. Then, it isdesirable that the amount of hydrogen consumed at the first step beminimized. Therefore, the present inventors have conducted intensiveresearch and achieved the following invention.

Namely, in removing carbon monoxide from a hydrogen-containingtreatment-object gas, the treatment-object gas is exposed to a firstmetal catalyst capable of methanating carbon monoxide so as to removethe carbon monoxide as methane. In doing this, it is preferred that themethanation reaction be effected with setting the methanation reactiontemperature higher than 160° C. and lower than 240° C.

With this, by setting the temperature higher than a predeterminedtemperature (higher than 160° C.), the methanation reaction will proceedto a certain degree, whereas by setting also this temperature lower thana predetermined temperature dower than 240° C.), it is possible tosufficiently restrict occurrence of methanation of carbon dioxide whichtends to involve consumption of hydrogen. More preferably, theupper-limit temperature is set at 200° C.

In this case, such relatively low temperature range is employed for themethanation reaction. Therefore, it is preferred from the view point ofcatalyst reactivity, the first metal catalyst comprise catalystcontaining Ru.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view showing a first embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An example of mode of using a carbon-monoxide removing system accordingto the present invention will be described.

From a carbon-monoxide shift converter reactor, a treatment-object gaswhich contains a relatively large amount, i.e. about 6000 ppm to 1 wt.%, of carbon monoxide is guided to a first CO remover (incorporating a“high quantity metal supported catalyst”). Generally, thistreatment-object gas contains no oxidizing component (oxygen).

Next, the treated treatment-object gas discharged from the first COremover is guided to a second CO remover. In this second CO remover(incorporating a “low quantity metal supported catalyst”), to a traceamount of carbon monoxide which remains un-removed at the first COremover, air or oxygen alone is added as an oxidizing agent to obtain[O₂]/[CO] ratio of 0.5 to 4.5 and then a reaction is carried out at arelatively-low temperature range. In this, a CO sensor or the like maybe provided between the second CO remover and the first CO remover sothat the amount of oxidizing agent be controlled based on a detectionvalue from this sensor.

The “high quantity metal supported catalyst” is a catalyst comprising0.5 to 5 wt. % of one or more kinds of metal selected from the groupconsisting of Ru, Pt, Rh, and Pd supported on an alumina. The “lowquantity metal supported catalyst” is a catalyst comprising 0.1 to 2 wt.% of one or more kinds of metal selected from the group consisting ofRu, Pt, Rh and Pd supported on an alumina support.

In each catalyst reaction, a value of GHSV (Gas Hourly Space Velocity:treatment-object gas flow amount/catalyst volume (1/h)) is set to about500 to 100000/h (set to a practically possible range).

Further, the reaction temperature (° C.) at the first CO remover is setto a range from 155 to 300° C. Whereas, the reaction temperature at thesecond CO remover is set to a range from 50 to 250° C. lower than thereaction temperature at the first CO remover. That is, the former is setto be higher than the latter.

In the above, preferably, the reaction temperature of the first step atthe first CO remover is set to 155 to 300° C. (more preferably, to 175to 250° C.). And, preferably, the reaction temperature of the secondstep at the second CO remover is set to a relatively lower range of 50to 250° C. (more preferably, to 100 to 160° C.). This is because thetemperature range should differ in correspondence with each object.

If the temperature of the first step is lower than 155° C., themethanation activity tends to be lower. Whereas, if it is higher than300° C., an influence of a side reaction tends to appear. Then, if thereaction (incorporating a “low quantity metal supported catalyst”) to atrace amount of carbon monoxide which remains un-removed at the first COremover, air or oxygen alone is added as an oxidizing agent to obtain[O₂]/[CO] ratio of 0.5 to 4.5 and then a reaction is carried out at arelatively low temperature range. In this, a CO sensor or the like maybe provided between the second CO remover and the first CO remover sothat the amount of oxidizing agent be controlled based on a detectionvalue from this sensor.

The “high quantity metal supported catalyst” is a catalyst comprising0.5 to 5 wt. % of one or more kinds of metal selected from the groupconsisting of Ru, Pt, Rh, Pd and Ni supported on an alumina. The “lowquantity metal supported catalyst” is a catalyst comprising 0.1 to 2 wt.% of one or more kinds of metal selected from the group consisting ofRu, Pt, Rh and Pd supported on an alumina.

In each catalyst reaction, a value of GHSV (Gas Hourly Space Velocity:treatment-object gas flow amount/catalyst volume (1/h)) is set to about500 to 100000/h (set to a practically possible range).

Further, the reaction temperature (° C.) at the first CO remover is setto a range from 155 to 300° C. Whereas, the reaction temperature at thesecond CO remover is set to a range from 50 to 250° C. lower than thereaction temperature at the first CO remover. That is, the former is setto be higher than the latter.

In the above, preferably, the reaction temperature of the first step atthe first CO remover is set to 155 to 300° C. (more preferably, to 175to 250° C.). And, preferably, the reaction temperature of the secondstep at the second CO remover is set to a relatively lower range of 50to 250° C. (more preferably, to 100 to 160° C.). This is because thetemperature range should differ in correspondence with each object.

If the temperature of the first step is lower than 155° C., themethanation activity tends to be lower. Whereas, if it is higher than300° C., an influence of a side reaction tends to appear. Then, if thereaction temperature is set to be lower than 250° C., methanation ofcarbon dioxide which is unnecessary in the present invention, can berestricted in particular. For achieving the restriction of methanationof carbon dioxide and promotion of methanation of carbon monoxide, aneven more preferred range is a temperature range from 160° C. to 240° C.

On the other hand, at the second step, if the temperature range is setto be relatively low dower than 250° C.), oxidation reaction will mainlytake place, so that it becomes easier to reduce carbon monoxide to asufficient level.

If the temperature of the second step is lower than 50° C., thereactivity will be low. Whereas, if it is higher than 250° C., it mayhappen that it becomes difficult to reduce carbon monoxide to be lowerthan several tens of ppm, due to a side effect such as a reverse-shiftreaction (reverse water-gas shift reaction).

With the above arrangements, at the first CO remover, carbon monoxide isreacted with hydrogen in the treatment-object gas to be converted intomethane at the relatively high temperature range according to a reactionformula: CO+3H₂→CH₄+H₂O, so that most of the carbon monoxide may beremoved. This reaction can take place through appropriate control of thecatalyst reaction temperature, substantially without aid of an oxidizingagent. In this case, the amount of carbon monoxide removable by themethanation reaction can be higher than 70% of that introduced into thefirst CO remover.

Next, at the second CO remover, mainly through a oxidation reactioninvolving an oxidizing agent according to a formula: 2CO+O₂→2CO₂, carbonmonoxide is removed. This removal is possible to a level of several tensof ppm (preferably, 10 ppm) or lower. Hence, this may be suitablyapplied to a polymer electrolyte fuel cell.

Accordingly, for carbon monoxide contained in a treatment-object gas ata reaction outlet of a carbon monoxide shift converter, the conventionalmethod requires oxygen three times in the mole ratio, i.e. 6chemicalequivalents. On the other hand, according to the present invention, onlywith addition of air containing oxygen by a concentration lower than thechemical equivalent of the carbon monoxide, the carbon monoxidecontained in the treatment-object gas may be removed. And, unnecessaryconsumption of hydrogen may be reduced correspondingly.

Further, even when air is selected as the oxidizing agent, the additionamount of air is small. Thus, the amount of nitrogen to be mixed intothe treatment-object gas too can be reduced. Consequently, reduction inthe partial pressure of the hydrogen in the treatment-object gas may bedecreased.

By appropriate control of the reaction temperatures of the first COremover and the second CO remover, it is possible to restrict such sidereaction as: CO₂+4H₂→CH₄+2H₂O or CO₂+H₂→CO+H₂O, etc. carbon monoxide maybe removed very efficiently even when several tens of % of carbondioxide is co-existent in the gas and the loss of hydrogen may bereduced.

The removing method of the invention is very suitable also for a casewhere methane is present in the treatment-object gas, since the methodfunctions well in such case as well.

Further, comparing the reaction temperature of the first CO remover andthe reaction temperature of the second CO remover to each other, thereaction temperature shifts from a high temperature to a low temperaturealong the flow passage.

Moreover, if the treatment-object gas is supplied to a low-temperatureoperating fuel cell such as a solid polymer electrolyte fuel cell byemploying the method of the present invention, it is possible to supplyfuel gas with lower efficiency reduction, by avoiding CO poisoning ofthe electrode catalyst of the fuel cell.

FIG. 1 shows a construction of a system according to a first embodimentof the present invention for removing carbon monoxide in atreatment-object gas. Fuel 1 consisting mainly of natural gas isintroduced to a desulfurizer 2 to have its sulfur content removed. Next,this together with water vapor 3 is fed to a reformer 4 to be subjectedto a reforming reaction. Subsequently, it is subjected to acarbon-monoxide shift reaction (water-gas shift reaction) at acarbon-monoxide transformer 5.

After this unit, there are disposed a first CO remover 6 and a second COremover 7. To the second CO remover 7, air 8 is added as an oxidizingagent.

The first CO remover 6 is equipped with a first-catalyst reactioncondition setting mechanism 6 a for realizing a catalyst reactioncondition at this remover suitable for this invention. Thisfirst-catalyst reaction condition setting mechanism 6 a provides a flowcontrol function for controlling the amount of reformed gas passing theremover 6 in relation with the amount of catalyst available and atemperature controlling function for controlling the reactiontemperature, so that GHSV and the reaction temperature can be adjustablyset. On the other hand, the second CO remover 7 is equipped with asecond-catalyst reaction condition setting mechanism 7 a which providesan equivalent function to the first-catalyst reaction condition settingmechanism 6 a to the second remover 7. In general, in actual use, the SVvalue is fixed in a catalyst reaction. Therefore, the first-catalystreaction condition setting mechanism 6 a and the second-catalystreaction condition setting mechanism 7 a should be able to adjusting atleast the temperature of the respective catalyst reaction portion.

The second CO remover 7 is further equipped with an oxidizing-agentadding mechanism 7 b capable of adding an oxidizing agent withadjustment of its addition amount. The amount of this addition is set tobe an oxidizing-agent amount just sufficient for the oxidation inrelation with the CO concentration of the reformed gas at the entranceof the second CO remover 7. Needless to say, the oxidizing agent shouldnot be supplied excessively.

Next, modes of using this system will be described.

[First Embodiment]

The first CO remover 6 was charged with catalyst (as a first metalcatalyst and also as a high quantity metal supported catalyst)comprising 1 wt. % of ruthenium supported on granular alumina. Then, areformed gas (humidified gas containing 6000 ppm of carbon monoxide,5000 ppm of methane, 20% of carbon dioxide and 78.9% of hydrogen)obtained from the exit of carbon-monoxide shift converter 5 wasintroduced to this first CO remover 6, in which a methanation reactionof CO was effected at GHSV 3750-15000/h and at a temperature of 200 to230° C.

Next, the second CO remover 7 was charged with catalyst (as a secondmetal catalyst and also as a low quantity metal supported catalyst)comprising 0.5 wt. % of ruthenium supported on granular alumina. Thereformed gas obtained from the exit of this first CO remover 6 wasintroduced to the second CO remover 7. In this, air 8 containing oxygenby an amount corresponding to a ratio of 1.5 of [O₂]/[CO] relative tothe CO concentration of the reformed gas at the entrance of the secondCO remover 7 was added, so that CO oxidation was effected at GHSV15000/happroximately and at 150° C.

The results are summarized and shown in Table 1 below.

TABLE 1 first CO exit CO remover exit CO concentration of temperature SVvalue concentration second CO example (° C.) 1/h (ppm) remover (ppm) 1200 3750 353 0 2 200 5000 1627 0 3 210 5000 322 0 4 210 7500 1497 0 5230 7500 223 0 6 230 15000 889 0

In the above, the CO concentration of the treatment-object gasintroduced into the first CO remover was 6000 ppm. In the table above,as the value “0” as the CO concentration, the detection limit of COconcentration was 5 ppm.

Now, the concentrations of methane formed at the first CO remover therespective examples were as follows.

exit CO concentration of first CO remover concentration methane formedexample temperature (° C.) (ppm) (ppm) 1 200 353 7013 2 200 1627 4483 3210 322 7179 4 210 1497 4577 5 230 223 10145 6 230 889 10803

The results show that CO removal was possible in each case with anamount of oxygen (the amount of oxidizing agent) smaller than thechemical equivalent of the amount of carbon monoxide entering the firstCO remover

[Second Embodiment]

The first CO remover 6 was charged with catalyst (as a first metalcatalyst and also as a high quantity metal supported catalyst)comprising 2 wt. % of ruthenium supported on granular alumina. Then, areformed gas humidified gas containing 6000 ppm of carbon monoxide, 5000ppm of methane, 20% of carbon dioxide and 78.9% of hydrogen) obtainedfrom the exit of carbon-monoxide shift converter 5 was introduced tothis first CO remover 6, in which a methanation reaction of CO waseffected at GHSV 3750-5000/h and at a temperature of 220 to 260° C.

Next, the second CO remover 7 was charged with catalyst (as a secondmetal catalyst and also as a low quantity metal supported catalyst)comprising 1 wt. % of ruthenium supported on granular alumina. Then, thereformed gas from the exit of the first CO remover 6 was introduced tothis second CO remover 7, in which with addition of air containingoxygen by an amount corresponding to [O₂]/[CO] ratio of 1.3 relative tothe CO concentration of the reformed gas at the entrance to this secondCO remover 7, CO oxidation reaction was carried out at GHSV 15000/happroximately and at a temperature of 135° C.

The results are summarized and shown in Table 2 below.

TABLE 2 first CO exit CO remover exit CO concentration of temperature SVvalue concentration second CO example (° C.) 1/h (ppm) remover (ppm) 1220 3750 1021 0 2 220 5000 1510 0 3 240 3750 803 0 4 240 5000 965 0 5260 5000 1053 0

In the above, the CO concentration of the treatment-object gasintroduced into the first CO remover was 6000 ppm. In the table above,as for the value “0” shown as the CO concentration, the detection limitof CO concentration was 5 ppm.

In this example too, substantially same results as the above examplewere obtained regarding the methane formed.

The results show that CO removal was possible in each case with anamount of oxygen (the amount of oxidizing agent) smaller than thechemical equivalent of the amount of carbon monoxide entering the firstCO remover 6.

As described above, after the carbon-monoxide shift converter 5, thereare provided the two stages of CO removers, i.e. the first CO remover 6and the second CO remover 7. At the first CO remover 6, a major portionof CO contained in reformed gas is removed through its methanation. Atthe second CO remover 7, the remaining portion of CO is removed throughits oxidation with addition of a trace amount of oxidizing agent.Consequently, it was possible to remove carbon monoxide from reformedgas while significantly reducing the amount of oxidizing agent to beadded.

COMPARISON EXAMPLE 1

In this comparison example 1, catalyst comprising 2 wt. % of rutheniumsupported on granular alumina was charged into a CO remover, into whicha hydrogen balance gas having a carbon monoxide concentration of 6000ppm, a carbon dioxide concentration of 20% and a methane concentrationof 5000 ppm was added air containing 21% of oxygen (the ratio betweengas and air: [O₂]/[CO]=2 approximately), and then this mixture wasintroduced at GHSV 5000/h at the reaction temperature of 150° C. Whenoxidation removal alone was conducted under this condition, it was foundthat 33 ppm of CO remain un-removed.

COMPARISON EXAMPLE 2

In this comparison example 2, catalyst comprising 2 wt. % of rutheniumsupported on granular alumina was charged into a CO remover, into whicha hydrogen balance gas having a carbon monoxide concentration of 6000ppm, a carbon dioxide concentration of 20% and a methane concentrationof 5000 ppm was introduced at GHSV 5000/h at the reaction temperature of150° C. When the carbon monoxide removal by methanation was conductedunder this condition, it was found that only about 100 ppm of the carbonmonoxide was methanated.

OTHER MODES OF EMBODYING THE INVENTION

-   (a) In the foregoing embodiments, the system includes the    desulfurizer 2 and the carbon monoxide shift converter 5. Depending    on the kind of fuel, however, they system may eliminate these. That    is to say, the present invention does not provide any limitations in    the process of forming the reformed gas before the carbon monoxide    removing system.

In the above respect, however, the reformed gas should contain hydrogento be used as a fuel gas as its main component thereof (more than about50% on the dry basis) and also carbon monoxide to be removed.

In general, such reformed gas hardly contains such components as oxygenwhich is an oxidizing agent component.

-   (b) In the foregoing embodiments, air and oxygen are cited as    examples of the oxidizing agent. Alternatively, the oxidizing    element may be any. substance containing a certain component which    can contribute to oxidation.-   (c) In the foregoing embodiments, the first CO remover and the    second CO remover are provided separately from each other. Instead,    it is possible to provide, as a construction for effecting the    above-described process, a single-container construction housing a    catalyst for methanation disposed on the upstream side in the flow    direction of the treatment-object gas having catalyst for    methanation, a catalyst for oxidation disposed on the downstream    side in the same direction with a mechanism for introducing an    oxidizing agent to this portion.

In such case, the upstream portion of the container corresponds to thefirst CO remover and the downstream portion thereof corresponds to thesecond CO remover.

-   (d) Other embodiments of the present invention will be described    next. [Other Embodiments]

The first CO remover 6 was charged with catalyst (as a first metalcatalyst) comprising a granular alumina supporting 1 wt. % of rhodium.Then, a reformed gas (same as first and second embodiments) obtainedfrom the exit of carbon-monoxide shift converter 5 was introduced tothis first CO remover 6, in which a methanation reaction of CO waseffected at GHSV 3750-7500/h and at a temperature of 260 to 300° C.

Next, the second CO remover 7 was charged with catalyst (as a secondmetal catalyst and also as a low quantity metal supported catalyst)comprising 1 wt. % of ruthenium supported on granular alumina. Then, thereformed gas from the exit of the first CO remover 6 was introduced tothis second CO remover 7, in which with addition of air 8 containingoxygen by an amount corresponding to [O₂]/[CO] ratio of 1.3 relative tothe CO concentration of the reformed gas at the entrance to this secondCO remover 7, CO oxidation reaction was carried out at GHSV 15000/happroximately and at a temperature of 135C.

The results are summarized and shown in Table 3 below.

TABLE 3 first CO exit CO remover exit CO concentration of temperature SVvalue concentration second CO example (° C.) 1/h (ppm) remover (ppm) 1260 3750 1502 0 2 280 7500 1753 0 3 280 5000 728 0 4 300 7500 552 0 5300 5000 217 0

In the above, the CO concentration of the treatment-object gasintroduced into the first CO remover was 6000 ppm In the table above, asfor the value “0” shown as the CO concentration, the detection limit ofCO concentration was 5 ppm.

In this example too, substantially same results as the above examplewere obtained regarding the methane formed.

The results show that rhodium can be used also.

Further, by using the combination shown in the second embodiment of thefirst CO remover 6 comprising the ruthenium catalyst and the second COremover 7 comrpising the ruthenium catalyst and under the conditions ofexample 1 (the example shown as example 1 in Table 2), the catalyst ofthe second CO remover 7 was replaced by 0.5 wt. % of platinum supportedon granular alumina (an example of second metal catalyst). Then, thesystem was operated.

The operating conditions of the second CO remover 7 were: the catalystreaction temperature 170° C.; GHSV 30000 and the addition amount of air:[O₂]/[CO]=2.7. With these, the carbon monoxide concentration was reducedto 0 ppm (below the actual detection limit). Therefore, platinum can beemployed in the second CO remover in this invention.

Further, by using the combination shown in the first alternateembodiment of the first CO remover 6 composing the rhodium catalyst andthe second CO remover 7 comprising the ruthenium catalyst and under theconditions of example 5 (the example shown as example 5 in Table 3), thecatalyst of the second CO remover 7 was replaced by 1 wt. % of rhodiumsupported on granular alumina (an example of second metal catalyst).Then, the system was operated.

The operating conditions of the second CO remover 7 were: thetemperature 250° C.; GHSV 15000 and the addition amount of air:[O₂]/[CO]=4. With these, in this case too, the carbon monoxideconcentration at the exit of the second CO remover 7 was reduced to 0ppm (below the actual detection limit). Therefore, rhodium can beemployed in the second CO remover in this invention.

According to the present invention, the amount of oxidizing agent suchas air or oxygen to be added in the course of removal of carbon monoxidefrom a reformed gas can be reduced significantly. Thus, CO removal ofreformed gas is possible with minimizing loss of hydrogen by combustion.

Further, since in a fuel cell system methane produced by methanation ofcarbon monoxide can be used as a fuel for a burner of the reformer, itis possible to improve high efficiency of the system.

For this reason, it is possible to feed a fuel reformed gas with highefficiency to a low-temperature operating type fuel cell such as polymerelectrolyte fuel cell using a fuel such as natural gas, methanol, etc.

Since the above-described method of the present invention allowsefficient removal of carbon monoxide with a relatively high GHSV, themethod provides the advantage of allowing the CO removers to be formedcompact.

1. A method of removing carbon monoxide from a hydrogen-containingtreatment-object gas containing hydrogen as its major component andcarbon dioxide, the method comprising: a first step of causing thetreatment-object gas to contact a first metal catalyst comprising one ormore kinds selected from the group consisting of Ru, Pt, Rh, and Pd andcapable of methanating carbon monoxide at a temperature wheremethanation of carbon monoxide takes place by the first metal catalystso that a portion of the carbon monoxide is removed through carbonmonoxide methanation and where methanation of the carbon dioxide isrestricted; and a second step of causing the treatment-object gas fromthe first step together with an oxidizing agent to contact a secondmetal catalyst capable of oxidizing carbon monoxide so that a remainingportion of carbon monoxide is removed mainly through carbon monoxideoxidation, wherein in the first step, a carbon monoxide concentration ofthe treatment-object gas is reduced to 30% or lower of an originalcarbon monoxide concentration of the treatment-object gas charged intothe first step.
 2. The method of removing carbon monoxide, according toclaim 1, wherein in the second step, the second metal catalystcomprising one or more kinds selected from the group consisting of Ru,Pt, Rh and Pd is employed; and in the second step, a catalyst reactionlayer is maintained at a temperature where oxidation of carbon monoxidetakes place by the second metal catalyst involving addition of anoxidizing agent.
 3. The method of removing carbon monoxide, according toclaim 1, wherein a total amount of the oxidizing agent supplied at thesecond step is below about 3 chemical equivalents in oxygen conversionrelative to an amount of carbon monoxide originally contained in thetreatment-object gas introduced in the first step.
 4. The method ofremoving carbon monoxide, according to claim 1, wherein a total amountof the oxidizing agent supplied at the second step is below the chemicalequivalent in oxygen conversion relative to an amount of carbon monoxideoriginally contained in the treatment-object gas introduced in the firststep.
 5. The method of removing carbon monoxide, according to claim 1,wherein said hydrogen-containing treatment-object gas comprises areformed gas supplied to a fuel cell as a fuel gas.
 6. The method ofremoving carbon monoxide, according to claim 1, wherein in the secondstep, the second metal catalyst comprising one or more kinds selectedfrom the group consisting of Ru, Pt, Rh and Pd is employed; and in thesecond step, a catalyst reaction layer is maintained at a temperaturewhere oxidation of carbon monoxide takes place by the second metalcatalyst involving addition of an oxidizing agent.
 7. The method ofremoving carbon monoxide, according to claim 1, wherein a total amountof the oxidizing agent supplied at the second step is below about 3chemical equivalents in oxygen conversion relative to an amount ofcarbon monoxide originally contained in the treatment-object gasintroduced in the first step.
 8. The method of removing carbon monoxide,according to claim 2, wherein a total amount of the oxidizing agentsupplied at the second step is below about 3 chemical equivalents inoxygen conversion relative to an amount of carbon monoxide originallycontained in the treatment-object gas introduced in the first step. 9.The method of removing carbon monoxide, according to claim 6, wherein atotal amount of the oxidizing agent supplied at the second step is belowabout 3 chemical equivalents in oxygen conversion relative to an amountof carbon monoxide originally contained in the treatment-object gasintroduced in the first step.
 10. The method of removing carbonmonoxide, according to claim 1, wherein a total amount of the oxidizingagent supplied at the second step is below the chemical equivalent inoxygen conversion relative to an amount of carbon monoxide originallycontained in the treatment-object gas introduced in the first step. 11.The method of removing carbon monoxide, according to claim 2, wherein atotal amount of the oxidizing agent supplied at the second step is belowthe chemical equivalent in oxygen conversion relative to an amount ofcarbon monoxide originally contained in the treatment-object gasintroduced in the first step.
 12. The method of removing carbonmonoxide, according to claim 6, wherein a total amount of the oxidizingagent supplied at the second step is below the chemical equivalent inoxygen conversion relative to an amount of carbon monoxide originallycontained in the treatment-object gas introduced in the first step. 13.The method of removing carbon monoxide, according to claim 1, whereinsaid hydrogen-containing treatment-object gas comprises a reformed gassupplied to a fuel cell as a fuel gas.
 14. The method of removing carbonmonoxide, according to claim 2, wherein said hydrogen-containingtreatment-object gas comprises a reformed gas supplied to a fuel cell asa fuel gas.
 15. The method of removing carbon monoxide, according toclaim 3, wherein said hydrogen-containing treatment-object gas comprisesa reformed gas supplied to a fuel cell as a fuel gas.
 16. The method ofremoving carbon monoxide, according to claim 4, wherein saidhydrogen-containing treatment-object gas comprises a reformed gassupplied to a fuel cell as a fuel gas.
 17. The method of removing carbonmonoxide, according to claim 6, wherein said hydrogen-containingtreatment-object gas comprises a reformed gas supplied to a fuel cell asa fuel gas.
 18. A method of operating a fuel cell system, where carbonmonoxide is removed from a hydrogen-containing treatment-object gascontaining hydrogen as its major component and carbon dioxide,comprising: a first step of causing the treatment-object gas to contacta first metal catalyst comprising one or more kinds selected from thegroup consisting of Ru, Pt, Rh, and Pd and capable of methanating carbonmonoxide at a temperature where methanation of carbon monoxide takesplace by the first metal catalyst so that a portion of the carbonmonoxide is removed through carbon monoxide methanation and methanationof the carbon dioxide is restricted; a second step of causing thetreatment-object gas from the first step together with an oxidizingagent to contact a second metal catalyst capable of oxidizing carbonmonoxide so that a remaining portion of carbon monoxide is removedmainly through carbon monoxide oxidation; and using methane produced atthe first step as a reforming fuel, wherein in the first step, a carbonmonoxide concentration of the treatment-object gas is reduced to 30% orlower of an original carbon monoxide concentration of thetreatment-object gas charged into the first step.